Sensor for coil defrost in a refrigeration system evaporator

ABSTRACT

A system for controlling the defrost cycle of an evaporator comprising a sensor in the coil of an evaporator or downstream of the coil, the sensor configured to determine changes in the liquid mass ratio of the refrigerant in the evaporator. The difference in liquid mass ratio relating to frost buildup on the outside of said evaporator. When the difference in liquid mass ratio reaches a predetermined amount, corresponding to an unsatisfactory frost buildup, a defrost cycle is initiated. When the liquid mass ratio returns to a value that corresponds to a defrosted evaporator, the defrost cycle is discontinued.

FIELD OF THE INVENTION

This invention relates primarily to industrial or commercialrefrigeration systems. Specifically, this invention relates to systemsfor detecting an accumulation of frost on an evaporator and initiating adefrost cycle when the accumulation of frost reaches unacceptablelevels.

BACKGROUND OF THE INVENTION

Conventional refrigeration systems achieve cooling by allowing arefrigerant such as ammonia or a fluorocarbon to evaporate in the coilsof an evaporator. As the refrigerant evaporates, it absorbs heat fromthe surrounding area. A fan or other air moving device is used to drawair through the evaporator so that heat is removed more effectively fromthe air in the space that is being refrigerated.

As the temperature in the evaporator is generally below the freezingpoint of water, water vapor in the air often condenses on the evaporatorcoils and solidifies as frost. The buildup of frost adversely affectsthe cooling efficiency of the evaporator due to two cooperating factors.First, frost is a thermal insulator. The thicker the frost layer on theevaporator coils, the less efficient the heat transfer between the airand the evaporator. In addition, the buildup of frost restricts the airflow through the evaporator coils. As a result, less air is cooled.Eventually, as frost builds up, the combined effects of reduced air flowand reduced heat transfer require that the evaporator be defrosted torestore cooling efficiency.

One method for defrosting evaporators in prior systems has been todefrost them automatically and periodically under timed control. Thetime between the defrost cycles is set by an operator based onexperience with the system.

Other prior systems have tried to initiate defrost cycles only when thefrost buildup is large enough to adversely impact the cooling efficiencyof the refrigeration system. In U.S. Pat. No. 4,123,792, a system isdescribed which measures the power consumed by an electric fan motorwhich draws air over the evaporator. The principle of operation of thissystem is that frost buildup on the evaporator impedes air flow. Asfrost builds, the motor works harder to drive the fan, and when aparticular set point for power consumption by the fan is reached, thesystem presumes that defrost is required and a defrost cycle isinitiated. Other systems, such as that shown in U.S. Pat. No. 4,400,949,also use information regarding fan motor power consumption but combinethat information with information regarding the temperature of therefrigerated space and the temperature of the unit cooler to determinewhether defrost is required.

Other frost detection systems such as those shown in U.S. Pat. Nos.4,045,971 and 4,232,528, employ photoelectric sensors to detect thelevel of frost buildup on an evaporator coil. The system in U.S. Pat.No. 4,831,833 uses an air velocity sensor in the air flow path todetermine whether defrost should be initiated.

Another prior art system senses the differences in air temperature oneach side of the evaporator in the refrigerated space as well as thetemperature of the refrigerant leaving the evaporator. The data from thesensors is processed to determine if there is a frost buildup requiringthe initiation of the defrost cycle.

SUMMARY OF THE INVENTION

The various prior art systems described above all suffer fromlimitations that the present invention are designed to overcome in orderto create a system that can measure more precisely when defrost isrequired. In this way the defrost cycle is only initiated when it isnecessary considering the operator's priorities with respect to powerconsumption, cooling efficiency and other factors.

The problem with prior art timed defrost systems is that the amount ofwater vapor in the air in the refrigerated area varies depending on anumber of factors. Some of these factors include the humidity in theenvironment surrounding the space being cooled, the number of times theaccess door to the refrigerated area is opened, and the duration of suchopenings. The temperature in the area being cooled, the temperature ofthe evaporator, the velocity of the air passing through the evaporatorand the evaporation of water from items stored in the cooled area, areall factors that also affect the rate of frost buildup. Usually, timeddefrost systems must be set for the severe conditions when frost willaccumulate most rapidly. When conditions are not so severe, there areunnecessary defrost cycles which waste energy and cost money.Conversely, if the timer is set for modest conditions, and actualconditions are more severe, then defrost cycles could be delayed beyondwhen they are needed thereby compromising system performance.

The problem with systems that initiate defrost cycles based on powerconsumption, such as that disclosed in the '792 and '949 patents, isthat factors other than frost buildup also impact the power requirementsfor a fan motor. Such factors include the supply voltage, thetemperature in the cooled space, and the age of the motor. The systemdescribed in the '949 patent also has the disadvantage that thecharacteristics of refrigeration system components vary with age andloss of refrigerant. Such a system cannot compensate for these factors.

The problem with frost detection systems that rely on photoelectricsensors, such as that disclosed in the '971 patent, is that they areonly capable of sensing frost at a particular location on an evaporator.As frost buildup is not always regular or uniform, frost may build atlocations away from the photoelectric sensor and not be detected. Thiswill cause the evaporator to operate inefficiently because defrostingmay be needed even though it is not detected due to the location of thesensors. In other situations frost may build up near the sensor to agreater extent than at other locations causing defrost to be initiatedwhen it is not needed. Another deficiency of such systems is that theymay not detect the buildup of transparent, clear ice. The system in the'833 patent suffers from similar location-dependent deficiencies.

The problem with systems that rely on temperature differences on eachside of the evaporator, and the temperature of the refrigerant as itleaves the evaporator, is that they are complex, and changes intemperature across the evaporator indicative of frost buildup may occurin other situations as well. In addition, such systems cannot compensatefor changes that occur with age or loss of refrigerant.

Accordingly, the inventors determined that there exists a need for afrost detection system that is more accurate, reliable and lessexpensive to implement than existing systems and which is unaffected bychanges in the system due to changes in system components, or age orloss of refrigerant.

The present invention is an improved method and system for detecting andpreventing the capacity reduction impact of frost building on a coilsurface. As discussed above, prior methods have relied on air sidepressure increase, surface frost optical detection, air side temperaturechange with time, fan power increase or other external measures thatindirectly indicate frosted coil performance reduction. This inventionrelies on detecting a change in the amount of internal refrigerantliquid that is evaporated by the heat exchanger, and/or changes in theratio of refrigerant liquid to refrigerant vapor. The invention may beused to initiate coil defrost in any evaporating refrigerant coolingsystem, including direct expansion and liquid overfeed evaporators.

In an overfeed evaporator coil, more liquid is introduced into the coilthan is evaporated by the coil. The excess liquid is called overfeed,which returns to the low pressure side accumulator. By overfeeding theevaporator, the inner surface is kept thoroughly wetted and thusachieves optimum heat transfer.

In an evaporating refrigerant cooling system, the ratio of liquidrefrigerant to evaporated refrigerant in the vapor phase is referred toas the liquid mass ratio. As the coil builds frost on its exterior, theevaporative efficiency declines, and as the evaporative efficiencydeclines, less refrigerant is evaporated, and the liquid mass ratioincreases. According to the invention, the liquid mass ratio is measuredwith a suitable sensor, including but not limited to a void fractionsensor. The sensor produces an output signal that is reflective of theamount of liquid in the refrigerant flow stream. When the system isfully defrosted, e.g., at start-up, or after a full defrost, the sensorand its control system can measure a first or initial or full defrostliquid mass ratio, and use that ratio as the starting point fordetermining the trigger point for a defrost cycle. As a coil buildsfrost, the liquid mass ratio increases. When the increase in liquid massratio exceeds a specified value, that is, a predetermined increase overthe first/initial/full defrost value, a control will signal that defrostof the coil is required. The system can initiate defrost automaticallyupon receipt of such signal, or can be configured to alert a systemoperator to manually authorize system defrost.

After the coil defrosts fully, the control system may optionally measurethe liquid mass ratio, compare it to a first/initial liquid mass ratioand/or to a previous full defrost liquid mass ratio, and optionally usethe new ratio, or optionally an average of prior full defrost liquidmass ratios, to use as the starting point for determining the triggerpoint for the next defrost cycle. In this way the system can be dynamicas it constantly adjusts to actual site and system conditions, and thustakes into account such factors as the age and possible loss ofrefrigerant. In the case of a liquid overfeed evaporator, the controlsystem can also use input from the liquid mass ratio sensor to detect ifan evaporator is operating at an optimum overfeed rate. The overfeedrate may not be optimum due to liquid feed valve settings or a reductionin heat transfer unrelated to frost on the coil.

The operator can manipulate the trigger point to meet specificrequirements based on system priorities. The defrost trigger point mightbe set low (e.g., when the liquid mass ratio is 5% over thefirst/initial/full defrost liquid mass ratio), when just a little bit offrost is starting to form, if high performance/efficiency (frostinhibits performance) is required. Alternatively, the defrost triggerpoint might be set higher if some capacity loss is acceptable and/orfewer defrost cycle events is desired.

According to one embodiment of the invention, there is provided a frostdetection system for an evaporator which senses frost buildup bymeasuring the liquid mass ratio in or exiting from the evaporator coil.According to a preferred embodiment of the invention, a liquid massratio sensor is located in the evaporator coil. According to anotherembodiment of the invention, a liquid mass ratio sensor is locatedbetween the evaporator coil and the compressor.

According to one embodiment of the invention, there is provided a frostdetection system which need not take into account temperature in therefrigerated area.

According to one embodiment of the invention, there is provided a frostdetection system that need not take into account changes in theoperating characteristics of the refrigeration equipment due to aging.

According to one embodiment of the invention, there is provided a frostdetection system that assumes that the heat load is constant. Accordingto another embodiment of the invention, the frost detection system maybe provided with a device that measures the heat load of the system, forexample the air temperature into the coil relative to coil saturationtemperature or the total flow rate of refrigerant (both liquid andvapor), and the heat load information is used to adjust the defrostpoint for specific liquid mass ratios detected by the liquid mass ratiosensor.

According to one embodiment of the invention, there is provided a frostdetection system that is more accurate, reliable and less expensive toimplement that existing systems.

According to one embodiment of the invention, there is provided a methodfor controlling and/or initiating the defrost cycle of an evaporativecoil having the following steps: detecting the ratio of liquidrefrigerant to refrigerant in a vapor phase; and initiating a defrostcycle when the ratio of liquid refrigerant to vapor phase refrigerantequals or exceeds a predetermined amount. The predetermined amount maybe changed according to operator preference. According to this and otherembodiments of the invention, a first ratio of liquid refrigerant tovapor phase refrigerant may be determined when said evaporative coil hasno frost. According to other embodiments, a defrost cycle may beinitiated when the detected liquid to vapor mass ratio is an amounthigher (e.g., 5%, 10%, 15%) than said first liquid to vapor mass ratio.

According to one embodiment of the invention, there is provided a methodfor controlling and/or initiating the defrost cycle of an evaporatorhaving the following steps: detecting a first capacitance betweencharged plates situated in the coil of an evaporator, or downstream ofthe coil; detecting a second capacitance between the charged plates; andinitiating a defrost cycle when a difference between the firstcapacitance and the second capacitance equals or exceeds a predeterminedamount. The predetermined amount may be changed according to operatorpreference. According to this and other embodiments of the invention,the difference between said first capacitance and said secondcapacitance corresponds to a difference in volumes of fluid passingbetween said charged plates. According to further embodiments of theinvention, the first capacitance is determined when said evaporator haslittle or no frost.

According to a preferred embodiment, the method is used in a liquidoverfeed evaporator, but it may also be used in other systems includingdirect expansion systems.

According to another embodiment of the invention, there is provided anapparatus for initiating coil defrost in an evaporator, the apparatusincluding a refrigerant evaporating heat exchange coil and a sensor fordetecting the ratio of liquid refrigerant to refrigerant in a vaporphase. Said sensor may be located in said coil, or between said coil anda condenser of said evaporator, more particularly between said coil anda compressor of said evaporator, and more particularly between said coiland a separator of said evaporator.

According to a preferred embodiment of the invention, the refrigerantevaporating heat exchange coil is in a liquid overfeed evaporator.

According to another embodiment of the invention, there is provided anapparatus for initiating coil defrost in a refrigeration system, theapparatus including a refrigerant evaporating heat exchange coil and aliquid mass ratio sensor located in the coil, or downstream of saidcoil, wherein said liquid mass ratio sensor is a capacitance sensor.According to this embodiment, the liquid mass ratio sensor may include aplurality (two or more) of spaced apart conductive elements conductivelyconnected to a current source. According to this embodiment, the sensordetects changes in capacitance due to changes in the amount of liquidbetween the spaced apart conductive elements. According to a furtherembodiment of the invention, the liquid mass ratio sensor is a parallelplate sensor. According to yet a further embodiment of the invention,the liquid mass ratio sensor is made of parallel plates configured toreceive a charge, and where the sensor is configured to take capacitancereadings that reflect a volume of liquid passing between the plates ofthe sensor. According to various other embodiments of the invention, theconductive elements may take the form of coils, cylinders, or othershapes. According to a preferred embodiment of the invention, theconductive elements of the sensor may be in the form of parallelconcentric cylinders.

According to another embodiment, a first part of the capacitance sensoris the metal wall of the pipe through which the refrigerant is passing,and a second part of the capacitance sensor is an electrode situated inthe interior of the pipe. According to a preferred aspect of thisembodiment, the internal electrode portion of the sensor consists of aplurality of parallel metal rods covered in an insulating material suchas PTFE (Teflon). The rods are electrically connected together on oneend. An electrical connection is made to an external electronics unitthrough an insulating pressure tight fitting. According to onesub-embodiment, the parallel rods are arranged in an arc spaced a radialdistance from the inside pipe surface by insulating spacers located ateach end of the rods. According to another sub-embodiment, the parallelrods are arranged in a plane across the interior space of the pipe. Thespacers and rods allow for free liquid flow under the rods (between therods and the inside surface of the pipe). Together, the rods and themetallic pipe make a capacitive type sensor that is responsive to theamount of refrigerant flowing between the rods and pipe wall.

DESCRIPTION OF THE DRAWINGS

The subsequent description of the preferred embodiments of the presentinvention refers to the attached drawings, wherein:

FIG. 1 shows a perspective view of a sensor according to an embodimentof the invention.

FIG. 2 shows an end view of the sensor shown in FIG. 1.

FIG. 3 shows a cross-sectional view of the sensor shown in FIGS. 1 and2.

FIG. 4 is a representation of a refrigerant evaporating cooling systemhaving a sensor according to an embodiment of the invention.

FIG. 5 shows an end view of a capacitance sensor electrode according toan embodiment of the invention.

FIG. 6 shows a cut-away perspective view of a capacitance sensorelectrode according to the embodiment shown in FIG. 5.

FIG. 7 shows a side perspective view of the capacitance sensor electrodeshown in FIGS. 5 and 6, removed from the pipe.

FIG. 8 shows an end view of a capacitance sensor electrode according toa different embodiment of the invention.

FIG. 9 shows a cut-away perspective view of a capacitance sensorelectrode according to the embodiment shown in FIG. 8.

FIG. 10 shows a side perspective view of the capacitance sensorelectrode shown in FIGS. 8 and 9, removed from the pipe.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of particular embodiments of the invention,set out to enable one to practice an implementation of the invention,and is not intended to limit the preferred embodiment, but to serve as aparticular example thereof. Those skilled in the art should appreciatethat they may readily use the conception and specific embodimentsdisclosed as a basis for modifying or designing other methods andsystems for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentassemblies do not depart from the spirit and scope of the invention inits broadest form.

FIG. 1 shows a sensor 2 according to one embodiment of the invention.The sensor shown in FIG. 1 works on the basis of capacitance change dueto the amount of liquid refrigerant between two charged plates. Asmentioned above, this is only one embodiment of the invention accordingto which the amount of liquid refrigerant in the coil or leaving thecoil may be determined according to any number of known methods.

According to the embodiment of FIG. 1, the capacitance sensor includescharged plates in the form of concentric cylinders, 6 and 8, see FIGS. 2and 3. The sensor shown in FIGS. 1-3 is a 2-inch HBDX-SAM-Mark voidfraction sensor (in gas-liquid two-phase flow, the void fraction isdefined as the fraction of the flow-channel volume that is occupied bythe gas phase or, alternatively, as the fraction of the cross-sectionalarea of the channel that is occupied by the gas phase). TheHBDX-SAM-Mark sensor may be purchased from HB Products of Denmark, butany sensor that detects capacitance change between charged elements dueto changes in the amount of liquid between them can be used according tothe capacitance detection embodiment of the invention. Cylinder 6 isheld in the refrigerant flow path of cylinder 8 (which may also serve asthe sensor housing) by stacks 12. Stacks 12 are conductively connectedto charged cylinders 6 and 8. As the liquid refrigerant quantityincreases, the capacitance increases. The capacitance change, which isvery small, is detected by a sophisticated electronic circuit 18 andthen output in a useable signal to control system 20. According to analternate embodiment, the sensor may include additional concentriccylinder 4, held in the refrigerant flow path of cylinder 8 by supports10, and capacitance changes between cylinders 4 and 6, between cylinders4 and 8, or between cylinders 4, 6 and 8 may be used to compare changesin the amount of liquid between them over time.

According to another embodiment, shown in FIGS. 5-7, a first part of thecapacitance sensor is the metal wall of the pipe through which therefrigerant is passing, and a second part of the capacitance sensor isan electrode 22 situated in the interior of the pipe. According to apreferred aspect of this embodiment, the internal electrode portion 22of the sensor consists of an array of a plurality of parallel metal rods24 covered in an insulating material such as PTFE (Teflon) supported insupport elements 26. According to an embodiment of the invention,support elements 26 are plastic, and the surface of the support elements26 that face the rods define a plurality of recesses 25 configured toreceive and hold the rod ends in fixed and spaced positions. The supportelements may be held together by a connecting rod 29 connecting thesupport elements and drawing them together, either by threading througha threaded hole 31 in the supports, or by passing through a hole in thesupports into a threaded nut, or by any other known method.

The rods are electrically connected together at one end. The rods may besolid or they may be hollow. The rods may have a cylindrical crosssection, as shown in FIG. 5, or they may have a cross-section having adifferent shape, including square, elliptical, pentagonal, hexagonal,etc. The rods may be arranged in an arc proximate to the inside surfaceof the pipe as shown in FIG. 5, or they may be arranged in a planeacross the interior space of the pipe as shown in FIGS. 5-7. Thediameter of the rods in the embodiment shown in FIG. 5 is 0.1875 inches,and the radial distance between the centerline of the metal portion ofthe rod and the inside of the pipe is about 0.375 inches. The spacingbetween the rods and the pipe affects sensitivity, flow thicknessmeasurement, and sensor output range. A closer rod-to-wall distanceincreases sensitivity but can affect the flow stream if the liquidimpacts the sensor structure, and can also decrease sensor output rangewith thicker liquids. Preferred spacing between the centerline of therod and the inside surface of the pipe is considered to be between 0.1inches and 0.5 inches, with more preferred spacing at 0.25 inches to0.45 inches, and most preferred spacing at 0.35 inches to 0.375 inches.Other spacing between rods and pipe surface may be used according todifferent sensitivity, liquid thicknesses, and sensor output rangeconsiderations and requirements.

The rods 24 shown in FIG. 5 have a length of about 10.5 inches, but theymay be of any convenient length. Longer rods make the capacitor platearea higher, which in turn increases the sensitivity of the sensor. Thenumber of rods shown in FIG. 5 was selected to cover about a third ofthe circumference of the pipe in which they are situated. The rods areelectrically connected to an external electronics unit with insulatedwires 27 through an insulating pressure tight fitting 30. The parallelrods of FIG. 5 are spaced a radial distance from the inside pipe surfaceby insulating spacers 28 located at each end of the rods. The spacersand rods allow for free liquid flow under the rods (between the rods andthe inside surface of the pipe). Together, the rods and the metallicpipe make a capacitive type sensor that is responsive to the amount ofrefrigerant flowing between the rods and pipe wall.

FIGS. 8-10 show an alternative embodiment of the invention according towhich the rods are arranged in a plane across the internal space of thepipe. FIGS. 8-10 show a larger number of smaller diameter pipes,according to another embodiment of the invention.

According to a preferred embodiment, the liquid mass ratio sensor of theinvention, whether a capacitance sensor or other liquid mass ratiosensor, may be placed in the coil of the evaporator 14 (see FIG. 4), orit may be placed downstream of the evaporator, for example at location16. The sensor orientation may be vertical, horizontal or some otherangle. Whatever the orientation, the sensor is preferably exposed to theliquid and vapor flow in the evaporator or downstream of the evaporator,and the sensor response is reflective of actual changes in the amount ofliquid refrigerant evaporated.

The user may select a particular sensor output for defrost initiationdepending on the cost of initiating a defrost cycle (cost of systemdown-time) relative to the savings gained through capacity increase as aresult of defrost. The selected point for defrost initiation may varywith evaporator application and to user sensitivity to cost and/orefficiency. It is estimated that the capacity reduction (loss of coolingpower/efficiency) due to frost effects can range from 5% to 25% or more.Thus, depending on costs of defrost versus importance of efficiency forparticular applications, the system of the invention may be set toinitiate a defrost cycle when the sensor detects a change in the liquidmass ratio of 5%, 10%, 15%, 20% or more, which may correspond toreductions in capacity of anywhere from 5% to 25%.

Having now set forth exemplary embodiments and certain modifications ofthe concept underlying the present invention, various other embodimentsas well as certain variations and modifications of the embodimentsherein shown and described will obviously occur to those skilled in theart upon becoming familiar with said underlying concept. It should beunderstood, therefore, that the invention may be practiced otherwisethan as specifically set forth herein.

1. A method for controlling the defrost cycle of an evaporator,comprising: detecting a first ratio of liquid refrigerant to refrigerantin a vapor phase at a location in said evaporator or downstream of saidevaporator by detecting a first capacitance between two electrodes;detecting a second ratio of liquid refrigerant to refrigerant in a vaporphase at said location at a different time by detecting a secondcapacitance between said two electrodes; initiating a defrost cycle forsaid evaporator when a difference between said first capacitance andsaid second capacitance equals or exceeds a predetermined amount;wherein said two electrodes comprise a first electrode comprising ametal portion of a pipe through which said refrigerant is flowing and asecond electrode comprising an array of metal rods arranged parallel toone-another inside said pipe, and electrically connected to one-another,but not touching one-another, and not touching the inside surface of thepipe, and wherein the distance between the metal rods and the insidesurface of the pipe is sufficient to allow refrigerant to freely flowbetween said rods and the inside surface of the pipe.
 2. A methodaccording to claim 1, further comprising detecting a third ratio ofliquid refrigerant to refrigerant at said location, and stopping adefrost cycle for said evaporator when said third ratio is the same orwithin a predetermined amount of said first ratio.
 3. A method accordingto claim 1, wherein said first ratio is determined when said evaporatorhas no frost.
 4. A method according to claim 1, wherein said differencebetween said first ratio and said second ratio corresponds to adifference in volumes of liquid passing said location.
 5. A methodaccording to claim 1, wherein said difference between said firstcapacitance and said second capacitance corresponds to a difference involumes of liquid passing between said charged plates.
 6. A methodaccording to claim 1, wherein said predetermined amount may be changedaccording to operator preference.
 7. An evaporating refrigerant coolingsystem comprising an evaporator coil, a liquid mass ratio sensor locatedin said coil or downstream of said coil, and a control system configuredto initiate a coil defrost cycle when said liquid mass ratio sensoroutputs a value that equals or exceeds a predetermined value and todiscontinue said defrost cycle when said liquid mass ratio sensoroutputs a second value that is equal to or less than a secondpredetermined value, said liquid mass ratio sensor comprisingspaced-apart conductive elements configured to receive a charge, saidsensor configured to take capacitance readings reflective of a volume offluid passing between said plates; wherein said spaced-apart conductiveelements comprise a first electrode comprising a metal portion of a pipethrough which said refrigerant is flowing and a second electrodecomprising an array of metal rods arranged parallel to one-anotherinside said pipe, and electrically connected to one-another, but nottouching one-another, and not touching the inside surface of the pipe,and wherein the distance between the metal rods and the inside surfaceof the pipe is sufficient to allow refrigerant to freely flow betweensaid rods and the inside surface of the pipe.
 8. An apparatus accordingto claim 7, wherein said metal rods are arranged in an arc, where eachmetal rod is configured to be the same distance from the inside surfaceof the pipe.
 9. An apparatus according to claim 7, wherein said metalrods are arranged in a plane across an interior of said pipe.
 10. Anapparatus according to claim 7, wherein said metal rods are supportedbetween two supports, each support defining a plurality of recesses in arod-facing surface, said recesses configured to receive and support saidrods and to keep them separated from one-another.
 11. A liquid massratio sensor comprising spaced-apart conductive elements configured toreceive a charge, said sensor configured to take capacitance readingsreflective of a volume of fluid passing between said conductiveelements; wherein said spaced-apart conductive elements comprise a firstelectrode comprising a metal portion of a pipe through which saidrefrigerant is flowing and a second electrode comprising an array ofmetal rods arranged parallel to one-another inside said pipe, andelectrically connected to one-another, but not touching one-another, andnot touching the inside surface of the pipe, and wherein the distancebetween the metal rods and the inside surface of the pipe is sufficientto allow refrigerant to freely flow between said rods and the insidesurface of the pipe.
 12. An apparatus according to claim 11, whereinsaid metal rods are arranged in an arc, where each metal rod isconfigured to be the same distance from the inside surface of the pipe.13. An apparatus according to claim 11, wherein said metal rods arearranged in a plane across an interior of said pipe.
 14. An apparatusaccording to claim 11, wherein said metal rods are supported between twosupports, each support defining a plurality of recesses in a rod-facingsurface, said recesses configured to receive and support said rods andto keep them separated from one-another.
 15. An electrode configured foruse in a liquid mass ratio sensor, said electrode comprising: an arrayof metal rods arranged parallel to one-another and electricallyconnected to one-another, but not touching one-another, wherein thedistance between the metal rods and the is sufficient to allowrefrigerant to freely flow between said.
 16. An electrode according toclaim 15, wherein said metal rods are arranged in an arc.
 17. Anelectrode according to claim 15, wherein said metal rods are arranged ina.
 18. An electrode according to claim 15, wherein said metal rods aresupported between two supports, each support defining a plurality ofrecesses in a rod-facing surface, said recesses configured to receiveand support said rods and to keep them separated from one-another.